1. Field of the Invention
The present invention relates to an image reading apparatus for reading out images on an image carrying paper sheet which is moved by a feeding motor. The present invention also relates to a controlling circuit for controlling such a feeding motor.
2. Description of the Related Art:
An image reading apparatus of the above-described type may be incorporated in a facsimile machine for example. As is well known, a facsimile machine incorporates various electronic devices, such as a CPU and a ROM, for controlling the operations of the machine.
The facsimile machine also includes a reading section and a recording section. The reading section is provided to read out images on an image carrying paper and for output of image signals corresponding to the images. To this end, the reading section is provided with a light source for illuminating the image carrying paper, and an image sensor for detecting the reflected light from the image carrying paper. The recording section is provided to print images on a recording paper sheet on the basis of the image signals from the reading section.
Typically, the reading section is provided with a stepping motor for feeding the image carrying paper sheet, and a motor driving unit for controlling the operation of the stepping motor on the basis of phase controlling data supplied by the CPU. When the phase controlling data are input to the motor driving unit, the stepping motor rotates through a predetermined angle.
Referring to FIG. 5, description will now be made to a color image reading procedure of the facsimile machine described above. For reading color images, three image-reading operations, which correspond to red (R), green (G) and blue (B) image signals, are performed for each readout request (A) supplied from the CPU.
Now, it is assumed that the readout request (A) is simultaneous with the input of the phase controlling data to the motor driving unit. In such an instance, the rotational speed of the stepping motor will vary as shown by the curved lines m in FIG. 5. In this case, as can be understood, the feeding amount or displacement of the image carrying paper will not be the same for the above-mentioned three reading operations. More specifically, the displacement of the image carrying paper is the greatest for the red image reading operation since the average speed of the motor is the greatest during the red image reading operation. On the other hand, the displacement of the paper is the smallest for the blue image reading operation since the average speed of the motor is the smallest during this operation. When the displacements of the image carrying paper are not equal for the three reading operations as described above, the image signals obtained by these operations will not represent the original images correctly.
However, the above problem is overcome by delaying the supply of the phase controlling data by a suitable time T with respect to the readout request (A). With such an arrangement, the rotational speed of the stepping motor will vary as shown by the curved lines n in the FIG. 5. As a result, the feeding displacements of the image carrying paper are equalized for the red, green and blue image reading operations.
Conventionally, the delayed supply of the phase controlling data is realized as follows. First, the CPU supplies a readout request (A) while simultaneously actuating a timer by software. Then, when a predetermined time (delay time) of the timer is up, the CPU writes the phase controlling data in the motor driving unit by interruption.
In the conventional method, however, there is a problem of failing to provide the accurate delay time (T) due to the above interruption. Specifically, when utilizing interruption, the interval between the occurrence of the interruption and the time when the CPU actually writes the phase controlling data in the motor driving unit is not always the same. Further, when there is another interruption which should be handled earlier than the interruption in question, the writing of the phase controlling data in the motor driving unit is further postponed.